Local variable in Posgresql - postgresql

Question

I'm moving from SQL Server to Postgresql. In SQL Server I have a code like this:
Declare #usr int = (select id from users where name = 'root');
Select * from info1 where usr = #usr;
Select * from info2 where usr = #usr;
Select * from info3 where usr = #usr;
I want to write such a code in Postgres. I found no local variables in Postgresql. I try this way:
create temporary table _usr on commit drop as
select id from users where name = 'root';
select * from info1 where usr = (select id from _usr);
select * from info2 where usr = (select id from _usr);
select * from info3 where usr = (select id from _usr);
The question is: Is where an any other way to save a local variables other than a temporary tables? May be I missing some type of objects in Postgresql?
PS. Data structure for this example:
create table users
(
id int not null,
name varchar(100)
);
create table info1
(
usr int not null,
i1a text,
i1b text
);
create table info2
(
usr int not null,
i2a text,
i2b text,
i2c text
);
create table info3
(
usr int not null,
i3a text,
i2b decimal(10),
i2c decimal(10),
i2d decimal(10)
);
insert into users values (1, 'root'), (2, 'admin');
insert into info1 values (1, 'one', 'first'), (1, 'two', 'second');
insert into info2 values (1, '1', '11', 't111'), (1, '2', '22', '222');
insert into info3 values (1, '1', 1, 10, 100), (1, '2', 2, 20, 200);
insert into info3 values (2, '3', 3, 30, 300);
UPDATE. The way with anonymous code block seems to be much uglier:
do $$
declare _usr int;
begin
_usr = (select id from users where name = 'root');
create temporary table _info1 on commit drop as
select * from info1 where usr = _usr;
create temporary table _info2 on commit drop as
select * from info2 where usr = _usr;
create temporary table _info3 on commit drop as
select * from info3 where usr = _usr;
end
$$;
select * from _info1;
select * from _info2;
select * from _info3;

Answer 1

There are no variables in SQL in PostgreSQL.
I think the best and simplest solution would be to repeat the (cheap) subquery:
SELECT info1.+
FROM info1
JOIN users ON users.name = info1.usr
WHERE users.name = 'root';
SELECT info2.+
FROM info2
JOIN users ON users.name = info2.usr
WHERE users.name = 'root';
SELECT info3.+
FROM info3
JOIN users ON users.name = info3.usr
WHERE users.name = 'root';
If the three tables have the same structure, you could UNION the queries and use a CTE:
WITH u AS (
SELECT id FROM users WHERE name = 'root'
)
SELECT info1.*
FROM info1
JOIN u ON u.id = info1.usr
UNION ALL
SELECT info2.*
FROM info2
JOIN u ON u.id = info2.usr
UNION ALL
SELECT info3.*
FROM info3
JOIN u ON u.id = info3.usr;